1. Field of the Invention
The present invention relates to sintered bearings and production methods therefor used in sustaining various rotational products and bearings for motors, for example, information devices, office equipment, electrical appliances, construction equipment, and the like.
2. Related Art
Sintered bearings produced according to a powdered metal sintering technique have been widely used for the above applications since they are easy and inexpensive to mass-produce, and in particular, because of their dimensional stability. Sintered bearings are roughly produced by compacting a metallic raw metallic powder into a bearing green compact, sintering the green compact into a sintered compact, then subjecting the sintered compact to a sizing process to finish into predetermined dimensions and shape.
Sintered bearings are commonly used as oil retaining sintered bearing in which lubricating oil is impregnated in pores therein for lubrication and dynamic pressure property. As lubricating oil retaining sintered bearings for accurate rotating and high speed rotating motors used in, for example, spindle motors for HDDs (hard disk drives), there have been provided bearings in which grooves for generating dynamic pressure are formed on an inner peripheral surface of the bearing. The dynamic pressure is generated by increase in the pressure of an oil film formed between a rotating shaft and the inner peripheral surface of the bearing, and this realizes sustaining of the rotating shaft at high rotational accuracy and bearing stiffness.
The lubricating oil impregnated bearings are not suitable for some kinds of products. That is the lubricating oil retaining sintered bearings sometimes exude the oil and contaminate the surroundings, and the bearings are not easy to use in portions required to usually be cleaned, such as hinges of covers in notebook-type personal computers and mobile telephones.
When lubricating oil retaining sintered bearings are used for guide bushes in construction equipments such as power shovels, the bearings are exposed to sand and water introduced therein. Therefore, bearings are, for example, sealed in cases to form waterproof means so as to extend the service life and to improve reliability. However, the waterproof means require complicated structures, and maintenance is not easy since the case must be opened when the lubricant must be resupplied. Moreover, when washing solvent is caught and retained in pores of sintered bearings during and after washing process, noxious gases are generated when increase of temperature in use.
Dry-type sintered bearings in which a solid lubricant such as molybdenum disulfide is dispersed in a metallic powder to obtain a solid lubricate may be used for products in which lubricating oil cannot be used. However, dry-type sintered bearings are corroded in high temperature and high moisture environments. Corrosion in the dry-type sintered bearing results in shortening of the service life in addition to wear and noise, and means for improving corrosion resistance have therefore been required.
Thus, conventional sintered bearings involve various problems such as leakage of lubricating oil, complicated structure for waterproof means, generating noxious gases, and poor corrosion resistance. In order to solve these problems, it may be proposed that the entire surface of a sintered bearing is coated by a resin. Specifically, the bearing is dipped into a resin solvent or the surface thereof is sprayed with a resin to form a resin coating thereover.
In the above proposed means, the resin coating over the surface of the bearing can avoid leakage of lubricating oil, generating noxious gases, and penetration of water, and can ensure corrosion resistance since the bearing is not exposed to ambient imbruement. However, such as the inner peripheral surface of the bearings are required to have high dimensional accuracy, and the above resin coating coverage is not suitable for accurate dimensions. That is, there are several difficulties in the resin coating process, in controlling and ensuring uniformity of coating thickness, such as these difficulties that resin coating cannot be formed in complicated bearing shape, and an extremely thin resin coating cannot be formed. Therefore, dimensional stability which is an advantage of sintered bearings, is deteriorated, in particular, the bearings are not suitable for small parts. Furthermore, the resin is difficult to enter into the small pores on the surface of the bearing, and the securing strength of the resin is low and the resin coating is stripped easily.